A discussion came up the other day that I've not been able to find a written answer to.
Does 121.5 MHz need to be monitored at every position in your unit, or is just having one NVCS with it selected sufficient?
This is more geared to towers, where I'm curious if both Air and Ground positions need 121.5 MHz selected? But I'm also wondering about what happens at th ACC too.

Anybody know where it's written what the protocol is?

In QM ACC we have 1 receiver/transmitter for VHF/UHF guard. It is monitored 24/7 at one sector only.

I couldn't tell you where it's written, but in practice at my tower, have it monitored in one position only.

In the towers I've worked in it's been monitored in one position only. The last place I worked was co located with an ACC and tower monitored it and one position in the ACC monitored it, If either us or the ACC couldn't monitor due to equipment problems we had to ensure the other unit was monitoring. That was local directive though, I don't recall seeing it anywhere else.